Heat oven to 375 degrees F. Line a 9- by 13-inch baking pan with parchment paper, leaving an overhang on two sides.

In a food processor, pulse the flour, brown sugar, baking powder, orange and lemon zests, salt, and 3/4 cup granulated sugar to combine. Add the butter and egg and pulse until crumbly (the mixture should look like wet sand). Reserve 2 cups of the mixture, then press the remaining mixture into the bottom of the prepared pan.

In a large bowl, toss the blueberries, lemon juice and remaining 1/4 cup granulated sugar. Spoon the blueberry mixture over the crust in the pan and crumble the remaining dough mixture over the top. Bake until golden brown, 40 to 45 minutes. Let cool completely in the pan.

Using the overhangs, transfer the crumb bar to a cutting board and cut into pieces.

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